1. Field of the Invention
This invention relates to a liquid crystal display device for a large scale display element, based upon an actuation principle of light scattering, and having low voltage driving properties and high contrast, and also to a process for producing the same. The liquid crystal display device of the present invention is applicable to light shutter, display board, etc.
2. Description of the Related Art
As liquid crystal display elements, those of twisted nematic (TN) mode and supertwisted nematic (STN) mode using nematic liquid crystals have been practically used. However, as to these elements, since polarizers are used, the display has low transparency, and further since an aligning treatment on the substrate is required, the viewing angle is narrow. Still further, the above elements have a drawback of difficultly providing a large display area and are expensive. Thus, in order to overcome these drawbacks, various light-scattering mode devices and the preparation thereof have been proposed.
Japanese patent application laid-open No. Sho 58-501631 and U.S. Pat. No. 4,435,047 disclose a process of producing a light-scattering device by dispersing a microencapsulated liquid crystal in a polymer such as polyvinyl alcohol and forming a film from the polymer. According to this process, the liquid crystal microencapsulated by polyvinyl alcohol present in the thin layer film and having a positive dielectric anisotropy, is aligned in the direction of the electric field, and the thin layer film becomes transparent in the case where the ordinary refractive index n.sub.o of the liquid crystal is equal to the refractive index n.sub.p of the polymer. When the electric field is removed, the direction of liquid crystal occupies random positions; hence the refractive index of the liquid crystal deviates from n.sub.o so that light-scattering occurs and the thin layer film becomes white-turbid and opaque. Based upon such an actuation principle, preparation of a liquid crystal device having a bright and good contrast has been regarded as possible.
Further, Japanese patent application laid-open No. Hei 1-198725 discloses a process for producing a light-scattering mode display device, which process comprises steps of mixing a liquid crystal with a monomer or an oligomer, preparing a light-modulating layer containing the mixture, and irradiating ultraviolet rays to the layer to provide a three dimensional network structure formed in the layer. The actuation principle in this process is similar to that of the above Japanese patent application laid-open No. Sho 58-501631, and it has been said therein that a three-dimensional network structure of the polymer is formed in the continuous layer of the liquid crystal so that the proportion of the liquid crystal in the light-modulating layer is so large that it is unnecessary to take any particular precaution to accord the refractive index of the liquid crystal with that of the polymer, to thereby effect a high contrast.
The process disclosed in the above Japanese patent application laid-open No. Sho 58-501631 has a drawback that the proportion of the polymer in the thin film-constituting material is larger than that of the liquid crystal therein, and for example, when an electric field is impressed thereto, the electric field is exerted onto the liquid crystal via the polymer; hence, a higher driving voltage is required in order to afford a change to the alignment of the liquid crystal molecules. Further, the process has another drawback that since the proportion of the liquid crystal is small and a clear threshold voltage is absent, it is difficult to practice a large scale display according to multiplex drive.
Further, in order to achieve a sufficient transparency when an electric field necessary for driving is impressed or when a temperature exceeding the clearing point is employed, there is an additional restriction in the aspect of material choice in that the refractive index of the liquid crystal should be similar to that of the polymer. Further, since the micro-encapsulation of the liquid crystal is accompanied with crushing and drying steps, additional time for the succeeding step of preparing a thin layer film is required and hence such steps are cumbersome.
In the case of the above Japanese patent application laid-open No. Hei 1-198725, a mechanical dispersion step and a polymerization step of the monomer or oligomer by way of heat or ultraviolet irradiation are required, and controls of these steps are very difficult. In other words, the process has a drawback that it is very difficult to uniformly form the three-dimensional network of a polymer having a size effective for producing light scattering. Further, in the case of a large area display as premise, it is very difficult to insert a mixture of a monomer or an oligomer with liquid crystal between the substrates. Further, a case may be present that a mixture of a monomer or an oligomer with a liquid crystal is coated onto one of the substrates, but in this case, a coating step and a laminating step of the substrates are required, and as a result, there occurs a drawback that the steps of preparing the device are very complicated. Further, in order to polymerize the monomer or oligomer, heat or ultraviolet rays should be irradiated; hence there remains a possibility that the characteristics of the mixed liquid crystal are varied.
These drawbacks bring about a serious obstacle when the liquid crystal device having a large area is practically used; hence they are problems to be overcome.
The present inventors have made extensive research in order to solve the problems, and as a result, they have invented a liquid crystal device having low voltage-driving properties and a high contrast for a large scale display element and being very simple and cheap, and a process for producing the same.